U.S. Pat. No. 4,510,052 shows a centrifugal separator, which is provided with a device of this kind to supply control liquid to its rotor in a manner described above to intermittently open and re-close a peripheral passage when needed or at equal time periods to empty the separator chamber of the rotor of all its contents of product or a wanted portion thereof. The passage is kept closed by pressing the valve slide in a direction closing the passage by a pressure force of a control liquid, which is present in a so called closing chamber, which is provided with an outlet valve at its outermost portion. This outlet valve is opened for discharge of more or less of the content of control liquid in the closing chamber by supplying a certain volume of control liquid by means of an injector driven by pressurized air at a high flow into a so called opening chamber, which is provided with a throttled outlet at its outermost portion. The control liquid present in the opening chamber influences the outlet valve with a pressure force in a direction opening the outlet valve against the action of a resulting force from a number of springs.
Gradually, the opening chamber is filled up with control liquid, which is entrained into the rotation of the rotor. The free liquid surface of the control liquid is then displaced radially inwardly and reaches rapidly a certain radial level, at which the pressure force from the control liquid accumulated in the opening chamber upon lasting conditions when the control liquid is fully entrained into the rotation of the rotor would exceed the resulting spring force and open the outlet valve whereby the control liquid would flow out of the closing chamber. However, the supply of control liquid is taking place at such a high flow that the opening chamber is filled up radially inwardly to a level far inside the first mentioned radial level with control liquid, which only partly has been entrained into the rotation of the rotor, before the outlet valve opens and lets control liquid out of the closing chamber.
When the outlet valve has opened this rapidly results in that so much control liquid has flowed out of the closing chamber that the pressure from the remaining control liquid in the closing chamber no longer is able to keep the passage closed against the liquid pressure from the product present in the separation chamber instead the passage is opened and contents of product flows out of the separation chamber. How large portion of the contents of the separation chamber is that flows out through the passage is determined by for how long time the control liquid present in the opening chamber, which gradually flows out through the throttled outlet, is able to keep the outlet valve open, that is the volume of and the flow at which control liquid is supplied to the opening chamber before the outlet valve opens.
In the known supplying device the amount is regulated at each discharge occasion by adjusting the time, during which the high air pressure is acting on the injector and presses control liquid out of the same. The whereabouts for the time in these connections is very short, which makes it very difficult to discharge a wanted volume of product at sufficient accuracy and repeatability.
The closing chamber and the injector are refilled with control liquid from a source of control liquid, which often consists of a tank, which is located at a limited level above the centrifugal separator or of the general water conduit system. The refilling of the product, which is to be separated in the separation chamber after a discharge has taken place cannot be done quicker than refilling of the closing chamber without having product leaking out through the passage. Since the supply of product to the separation chamber normally is not shut off during the discharge procedure the supply flow of control liquid to the closing chamber in these cases has to be regulated and adapted to the supply flow of the product to the separation chamber.
Since the available liquid pressure of the source of control liquid is low and, furthermore, if it consists of the general water-conduit system often is heavily varying, it is very difficult to adjust a suitable supply flow. The fact that the available liquid pressure is low also means that conduits and valve having large throughflow area have to be used, which makes it even more difficult to adjust a suitable supply flow.